1. Field of the Invention
The present invention relates to an optical space communication apparatus for making optical wireless communications between two distant places using a light beam.
2. Related Background Art
FIG. 1 shows the arrangement of a conventional optical space communication apparatus with an automatic tracking function. A light transmission/reception lens system 1 and movable mirror 2 are placed at a position opposing the partner apparatus, and a polarization beam splitter 3, transmission optical system 4, and semiconductor laser light-emitting element 5 are placed in turn in the incoming direction of the movable mirror 2. In the reflection direction of the polarization beam splitter 3, a beam splitting mirror 6, reception optical system 7, and 4-split photodetector 8 are placed. In the reflection direction of the beam splitting mirror 6, a reception optical system 9 and main signal photodetector 10 are placed.
The output from a multiplexer 11 is connected to the light-emitting element 5. The output from a pilot signal generator 12 for generating a pilot signal of a single frequency lower than that of the main signal for position detection is connected to the multiplexer 11, and the output from a main signal input terminal 14 to the multiplexer 11 via an amplifier 13. The output from the photodetector 10 is connected to a main signal output terminal 16 via an amplifier 15, the output of which is fed back via a detecting circuit 17. The output from the 4-split photodetector 8 is connected to a control circuit 19 via four amplifiers 18a to 18d, and the output from the control circuit 19 is connected to the movable mirror 2 via two movable mirror drivers 20a and 20b. Note that the movable mirror 2 is rotated in two directions about axes perpendicular to and parallel to the plane of paper of FIG. 1.
A main signal to be transmitted input from the input terminal 14 is amplified by the amplifier 13, and is multiplexed with a pilot signal from the pilot signal generator 12 by the multiplexer 11. The main signal is then converted into an optical signal by the light-emitting element 5. The output light coming from the light-emitting element 5 is polarized light, the plane of polarization of which is parallel to the plane of paper of FIG. 1, and is transmitted through the transmission optical system 4 and polarization beam splitter 3. The main signal light is then reflected by the movable mirror 2, and is output from the transmission/reception lens system toward the partner apparatus in the form of a light beam.
On the other hand, a received light beam coming from the partner apparatus enters the transmission/reception lens system 1, is reflected by the movable mirror 2, and then enters the polarization beam splitter 3. In this case, since this received light beam is polarized in a direction perpendicular to the plane of paper, it is reflected by the cemented surface of the polarization beam splitter 3, and travels in the direction of the beam splitting mirror 6. The received light beam is split into two directions by the beam splitting mirror 6. One light beam is reflected by the beam splitting mirror 6, and is focused on the main signal photodetector 10. On the other hand, the other light beam is transmitted through the beam splitting mirror 6, and is focused on the 4-split photodetector 8.
The optical signal is converted into an electrical signal by the main signal photodetector 10, and the electrical signal is amplified by the amplifier 15. The amplified signal is then output as a reception signal from the output terminal 16. At this time, the amplifier 15 receives a signal fed back from the detecting circuit 17 to attain automatic gain control.
Since the 4-split photodetector 8 has a low response speed, it has nearly no sensitivity to the high-frequency main signal, and detects the low-frequency pilot signal alone. A focused beam spot is formed on four photodetection portions 8a to 8d of the 4-split photodetector 8, and the outputs from these photodetection portions 8a to 8d are output to the control circuit 19 after being respectively amplified by the amplifiers 18a to 18d. The control circuit 19 sends drive signals to the movable mirror drivers 20a and 20b on the basis of the signals from the four detection portions 8a to 8d to drive the movable mirror 2, so that the focused beam spot position is located at the center of the 4-split photodetector 8, and outputs from the four detection portions 8a to 8d become equal to each other.
Since the positions of the light-emitting elements 5, 4-split photodetector 8, and main signal photodetector 10 are adjusted in advance to agree with each other on the optical axis, when the beam spot is formed at the center of the 4-split photodetector 8, the light beam is also focused at the central portion of the main signal photodetector 10. At this time, the transmission light beam originating from the light-emitting element 5 is normally output in the direction of the partner apparatus.
In this way, even when the angle of the apparatus has changed due to an external force, such as changes in temperature, and the like, the automatic tracking function effects, by moving, the movable mirror 2 to form the beam spot at the center of the 4-split photodetector 8, thus maintaining a satisfactory communication state without any deviation of the light beam from the direction of the partner apparatus.
However, in the aforementioned prior art, the beam splitting mirror 6 for splitting the received light beam and the 4-split photodetector 8, main signal photodetector 10, and light-emitting element 5 require precise positional adjustment. As a result, the optical system and its holding mechanism are complicated, and adjustments upon assembly become hard. In an electric circuit section, the transmitter requires the pilot signal generator 12 and multiplexer 11, and the receiver requires the amplifiers 18a to 18d corresponding to the photodetection portions 8a to 8d of the 4-split photodetector 8. Furthermore, adjustments for matching the characteristics of the four amplifiers 18a to 18d are required. In this way, the apparatus becomes expensive resulting from the automatic tracking function and its weight and size increase.
It is an object of the present invention to provide an optical space communication apparatus which can solve the aforementioned problems, can broaden the automatic tracking angular range by a simple arrangement, and has an inexpensive automatic tracking function.
In order to achieve the above object, an optical space communication apparatus according to the present invention is characterized by comprises transmission means for converting a first electrical signal into a first optical signal, and transmitting the first optical signal to a partner apparatus in the form of a first light beam, photodetection means for detecting a second optical signal by receiving a second light beam transmitted from the partner apparatus, and converting the second optical signal into a second electrical signal, and changing means for changing a transmission direction of the first light beam and a reception direction of the second light beam in a direction to maximize an intensity of the second electrical signal obtained by converting the second optical signal detected by the photodetection means.
Especially, the optical space communication apparatus according to the present invention is characterized in that a receivable angular range of the second optical signal is not more than a divergent angle of the first light beam to be transmitted.